This invention relates to an improved method of preparing high strength aluminum products from prealloyed aluminum powder.
The utilization of prealloyed aluminum powder in forming high strength aluminum products is well known. In the prior art, these products were characterized by superior strength and corrosion resistance in comparison with conventionally cast products but were usually deficient in ductility and toughness.
The improved properties of products of prealloyed powders are often the result of the high freezing rate of the powder particles formed during atomization. Due to the drastic chill of the molten particles, a large amount of alloying elements can be frozen into the aluminum matrix without the massive chemical segregation characteristic of conventionally cast products containing very high levels of alloying elements.
Prior art compositions of the prealloyed aluminum powder were generally of the precipitation-hardening or dispersion-hardening type aluminum alloys. Frequently dispersoid forming additions of transition elements, such as cobalt, iron, manganese, nickel, chromium, zirconium and the like, are added to the heat treatable alloys to improve the properties thereof. Typical compositions are set forth below.
__________________________________________________________________________ Alloy Compositions Fe Cu Mn Mg Cr Ni Zn Ti Co Zr V Al __________________________________________________________________________ -- 0-3 0-3 1.75-6.0 0-1.25 -- 5-13 -- 0-3 0-0.25 -- Bal.sup.1 .5-4.5 0-3 0-3 1.75-6.0 0-1.25 .5-6 6.5-13 -- -- -- -- Bal.sup.1 2-20 -- 0-10.sup.2 -- 0-10.sup.2 0-10.sup.2 0-10.sup.2 0-10.sup.2 0-10.sub.2 0-2.sup.2 Bal.sup.1 0-3 4-12 0-6 -- 0-2 -- -- -- -- 0-3 0-2 Bal.sup.1 __________________________________________________________________________ .sup.1 Balance aluminum and inconsequential amounts of other elements .sup.2 Total of these elements not to exceed 10%
In the prior art, a procedure which was found suitable for producing products from prealloyed aluminum powders is as follows:
1. Selecting a prealloyed powder of a particular size range, usually -100 mesh; PA1 2. Cold compacting to form a 70 to 90% dense green compact; PA1 3. Degassing the green compact to reduce contaminants, such as water vapor and H.sub.2 which are known to cause degradation of the final product; PA1 4. Hot compacting the green compact to full density; PA1 5. Hot working to the desired shape; and PA1 6. Solution heat treating and aging if the alloy is a precipitation-hardening type alloy.
The degassing treatment was normally conducted by exposing the powder or green billet to temperatures in the range of 900.degree. to 1050.degree.F either in an inert atmosphere or a partial vacuum. These conditions were necessary to reduce the contaminants of the powders to a level which would prevent the development of surface blisters and/or large amounts of internal porosity in the final product. Prior investigators have conducted degassing at intermediate temperatures (e.g., 750.degree.F) in an inert atmosphere, but they found the degassing procedure very inefficient in removing contaminants and poor mechanical properties in the resultant products.
Recent advances in the powder metallurgical art have improved substantially the tensile strength, elongation and stress corrosion resistance particularly with solutionn heat treated and aged Al-Zn-Mg type alloys. However, attendant with the outstanding properties was a relatively low fracture toughness.
Against this background, the present invention was developed.